1. Field of the Invention
The present invention relates generally to the fabrication of enclosures for electrochemical devices. More particularly, the present invention relates in one embodiment to sealing two portions of an electrochemical device enclosure to each other by a laser welding process.
2. Description of Related Art
Improving packaging efficiency of electrochemical devices such as electrochemical cells and capacitors is critical to many applications, especially for implanted devices. Hence, any space within the electrochemical device that can be utilized for active material contributes to improving the overall performance and longevity of the electrochemical device and, therefore, that of the implantable device that is being powered. For devices that are contained within enclosures comprised of two portions that are welded together, clearances must be provided in proximity to laser welds (or other fusion joining structures) to prevent thermally induced melt damage to internal components. Alternatively, insulators must be used which thermally protect the internal components of the electrochemical device. For example, a ribbed insulator which creates air pockets proximate to the welded seam within the device may be used. Such a ribbed insulator is disclosed in U.S. Pat. No. 6,933,074 to Frustaci et al., which is assigned to the assignee of the present invention and incorporated herein by reference.
The outside casing of a hermetically sealed battery or capacitor is typically stamped or deep drawn to its final shape and mated with a lid or second case half. The two “halves” must be sealed after the internal components are housed therein. The seam welding operation requires significant heat input at the interface of the casing halves, which produce temperatures within the casing that can damage the internal components. There also is a final close seal required after electrolyte is dispensed and the device is filled. Various welded seam configurations for electrochemical devices are described and shown in U.S. Pat. No. 6,613,474 to Frustaci et al., which is assigned to the assignee of the present invention and incorporated herein by reference.
U.S. Pat. No. 6,264,708 to Haraguchi et al., which is incorporated herein by reference, discloses a method of manufacturing a prismatic battery. A sealing plate is abutted to an upper open end of a prismatic case in which power generating elements are housed. Laser beams are moved along an abutment line parallel to the straight line of each side. In this manner, each side of the abutment line including each of the corners is laser welded, thereby hermetically sealing the prismatic case. In the method of Haraguchi et al., the entire length of each of the four sides is completely welded, corner-to-corner.
FIG. 1 is an illustration of the method and apparatus for laser welding a battery casing according to Haraguchi et al. In one embodiment, the opposed short sides 2 and 4 of a rectangular battery casing are welded as indicated by arrows 99A and 99B. Then, a first long side 6 of the rectangular casing is welded by laser 5 as indicated by arrow 98A, and then the second long side 8 of the battery casing is welded by laser 7 as indicated by arrow 98B to complete the joining of the sealing plate 3 to the prismatic case 1. Haraguchi et al. indicate that it is preferable to weld the opposed long sides 6 and 8 of the casing serially “so that thermal effects [of the welding] on the battery can be suppressed.” However, nowhere in the Haraguchi et al. patent is there shown or suggested a method according to the present invention in which short sections of a weld seam are sequentially welded using a plurality of welding passes around the perimeter of the cell. This is done in order to form the entire welded seam without causing adverse thermal effects on the internal battery components.